Biochar is a highly porous carbonized material that can be found on the soil after a forest has burned. The porous nature of biochar and other charcoal products has been found to provide a habitat for beneficial microbes that absorb toxins in the soil and convert organic detritus into useful materials for the growth of nascent plants.
Synthetic charcoal products and biochar can be made on an industrial scale by burning wood chips and other cellulosic materials in an oxygen deficient atmosphere. Biochar in particular has a remedial benefit on the soil due mainly to the highly porous nature of the charcoal it contains. These pores are able to absorb toxic metals and accommodate beneficial microbes that feed on the remaining organics, leaving the soil fit for plant growth.
Synthetic biochar is made and traded worldwide. It is used mainly for soil remediation and improved plant growth. Early manufacturing processes were essentially based upon those for making pure charcoal. The feedstock can be any cellulose containing material that will breakdown under anoxic conditions to produce charcoal. Wood chips are preferred. Although the cellulose in the wood decomposes mainly to carbon and water, at high temperatures, a side reaction converts some charcoal into biogases and bioliquids. As biochar is not a pure charcoal, it is sold at a lower price. The reaction by-products reduce the value further, as they are only marketable as cheap fuel.
The particles of synthetic biochar may be distributed on the soil with equipment used for other agricultural products, such as plant seed and pelletized fertilizer. However, since the charcoal in the biochar is somewhat friable, distribution using conventional agriculture equipment creates hazardous dust, and loss of useful product. Furthermore, the low bulk density and lack of particle sizing control of the biochar causes separation of any blend of biochar and plant seed and/or commercial fertilizer during handling and distribution. To overcome this problem, methods have been developed to protect the biochar particles with a layer of an inert ceramic material. This approach has been found to minimize product breakdown and increase bulk density. As the ceramic coating needs to be sintered at high temperature, undesirable by-products are formed at the expense of some of the charcoal. Also, the inert coating simply disintegrates into small particles that remain in the soil.
Ceramic coatings are typically applied as slurry to the cellulosic biomass feedstock as they are fed into the reactors. The reactors then convert the feedstock into biochar and sinter the ceramic coating in a single step. U.S. Pat. No. 5,944,960 teaches a system that includes a flame inside the reactor that is fed by biogas produced as a process by-product, to generate heat for the pyrolysis and ensure the absence of oxygen in the reaction zone.
Sintering the ceramic coating requires reactor temperatures of around 800° C. to 900° C., although the interior temperatures of the particles may be lower, due to the coating's inhibition of heat conduction and the porosity of the particles.
It has been established that a pyrolysis temperature of around 450° C. produces the highest porosity charcoal content in the biochar. At this temperature, by-product reactions are suppressed, leading to the maximum production of charcoal in the pellets. Therefore, a coating that can be sintered at a lower temperature range while not increasing production costs or complexity is desirable in the industry.
It should be noted that biochar may also be used in other industries. Biodiesel for sale as transportation fuel in Canada and the United States must meet strict quality guidelines (CAN/CGSB-3.524-2011 in Canada and ASTM 6751 in the U.S.). Biodiesel must have low water and glycerol content. Often biodiesel manufacturers must use post-manufacturing desiccants and absorptive resins to remove unwanted contaminants before the quality of the biodiesel is sufficient for sale. This is sometimes referred to as “polishing.” A biochar-based polishing agent would be advantageous because it is environmentally benign unlike some polymeric polishing agents. Thus, disposal of the bio-based based agent after polishing may be seen as having less of a negative impact. Because biochar is dusty and comprised of small particles that would contaminate the biodiesel, using un-pelleted biochar is not an option to absorb unwanted liquid contaminants such as water from transportation fuel. However, if biochar is densified into pellets that are robust and non-dusty, the product can be used as a polishing agent without introducing further contamination.
Biodiesel manufacturing is still most commonly performed using metal methoxide chemistry (e.g. sodium or potassium methoxide catalyzed transesterification). This manufacturing process requires that the feedstock material have very low free fatty acid content. Free fatty acids present in the feedstock react quantitatively with basic metal methoxides to form soap. Soap formation decreases yield and is an unwanted contaminant. Soap also causes and supports an emulsion between the organic and aqueous phases, which complicates phase separation and can diminish yield. The requirement for low free fatty acids in feedstock means that high quality or highly processed vegetable oils are used, which can be quite expensive in comparison to the selling price of the finished biodiesel product. Biodiesel manufacturing could be more profitable if lower quality feedstocks with high free fatty acid content, such as waste cooking oil, could be easily used. Two scenarios are possible considering conversion of high free fatty acid oils to biodiesel:
1) free fatty acids are removed or converted to alkyl esters (i.e. biodiesel). This allows the resulting oil mixture to be used in metal methoxide biodiesel production. This type of catalysis is most commonly associated with solid acid catalysts
2) direct conversion of the high FFA feedstock to biodiesel. This requires that the catalyst used catalyzes both esterification and transesterification reactions. This conversion can be done using heterogeneous liquid or gaseous acids such as sulfuric acid or hydrochloric acid. Neither is ideal because the finished product can carry acid and therefore a unit operation is required to remove acid from the biodiesel. A solid-acid catalyst obviates this concern because the acid moieties are bound to the solid catalyst and are not carried through to the final product.
Sulfonated biochar can catalyze esterification of free fatty acids to biodiesel in mixtures containing vegetable oil.
There are products on the market that can be used as solid acid catalysts for esterification of free fatty acids in oil. This procedure is generally known as “pre-esterification” because the esterification of the free fatty acids to biodiesel happens prior to the transformed mixture entering the usual metal methoxide biodiesel manufacturing process. Most solid acid catalysts for this pre-esterification procedure are sulfonated macroreticular cross-linked polyvinylchloride resins. That is, the catalyst is generally in the form of a small bead, which is made of cross-linked polyvinylstyrene. The morphology of the beads is quite rough, allowing the beads to have high surface area and therefore greater surface for catalysis. The active catalytic sites are pendant sulfonic (—SO3H) groups. A biochar catalyst is bio-based when considering disposing spent catalyst in a landfill.
A biochar catalyst that can be used in the above process would therefore be advantageous. Not only that, but a process for producing such a catalyst, with mechanical properties that allow its use in the biodiesel industry, would also be advantageous and desirable.